In this book we have asked expert researchers in the field of to provide chapters on different aspects of interaction between neighboring cells, in the context of cancer diseases. We have specifically focused our efforts on membrane-to-membrane contact-based rather than growth factors-mediated modes of intercellular communications. The contributing authors provide an extensive overview of their respective area of specialization, with an in-depth discussion of the molecular mechanisms of cell-cell interactions, the impact on tumor progression and response to therapies, as well as the cancer diagnostic value of this scientific information. This book aims to introduce essential aspects of the normal and pathological cellular fate and homeostasis to both scientists and clinicians, and also to provide established researchers with an update on the novelties and future directions this expanding field is witnessing.